An integrated multiple driver mesocosm experiment reveals the effect of global change on planktonic food web structure

Warming increases the compositional and functional variability of a temperate protist community

Phototrophic protists are a fundamental component of the world's oceans by serving as the primary source of energy, oxygen, and organic nutrients for the entire ecosystem. Due to the high thermal seasonality of their habitat, temperate protists could harbour many well-adapted species that tolerate ocean warming. However, these species may not sustain ecosystem functions equally well. To address these uncertainties, we conducted a 30-day mesocosm experiment to investigate how moderate (12 °C) and substantial (18 °C) warming compared to ambient conditions (6 °C) affect the composition (18S rRNA metabarcoding) and ecosystem functions (biomass, gross oxygen productivity, nutritional quality - C:N and C:P ratio) of a North Sea spring bloom community. Our results revealed warming-driven shifts in dominant protist groups, with haptophytes thriving at 12 °C and diatoms at 18 °C. Species responses primarily depended on the species' thermal traits, with indirect temperature effects on grazing being less relevant and phosphorus acting as a critical modulator. The species Phaeocystis globosa showed highest biomass on low phosphate concentrations and relatively increased in some replicates of both warming treatments. In line with this, the C:P ratio varied more with the presence of P. globosa than with temperature. Examining further ecosystem responses under warming, our study revealed lowered gross oxygen productivity but increased biomass accumulation whereas the C:N ratio remained unaltered. Although North Sea species exhibited resilience to elevated temperatures, a diminished functional similarity and heightened compositional variability indicate potential ecosystem repercussions for higher trophic levels. In conclusion, our research stresses the multifaceted nature of temperature effects on protist communities, emphasising the need for a holistic understanding that encompasses trait-based responses, indirect effects, and functional dynamics in the face of exacerbating temperature changes.

Responses of plankton community to threshold metal concentrations of cadmium and lead in a mesocosm experiment at Bay of Bengal

Metals are essential at trace levels to aquatic organisms for the function of many physiological and biological processes. But their elevated levels are toxic to the ecosystem and even brings about shifts in the plankton population. Threshold limits such as Predicted No Effect Concentration (PNEC - 0.6 μg/l of Cd; 2.7 μg/l of Pb), Criterion Continuous Concentration (CCC - 3.0 μg/l of Cd; 4.5 μg/l of Pb) and Criterion Maximum Concentration (CMC - 23 μg/l of Cd; 130 μg/l of Pb) prescribed for Indian coastal waters were used for the study. Short-term mesocosm experiments (96 h) were conducted in coastal waters of Visakhapatnam to evaluate responses of the planktonic community on exposure to threshold concentrations of cadmium and lead for the first time. Four individual experimental bags of 2500 L capacity (Control, PNEC, CCC & CMC) were used for the deployment and ambient water samples were analysed simultaneously to evaluate the impacts of the threshold levels in the natural waters. Chaetoceros sp. were dominant group in the control system whereas, Prorocentrum sp. Ceratium sp. Tintinopsis sp. Chaetoceros sp. and Skeletonema sp. were major groups in the test bags. Throughout the experiment the phytoplankton community did not show any significant differences with increased nutrients and plankton biomass (Chl-a <8.64 mg/m3). Positive response of plankton community was observed in the experimental bags. High abundance of diatoms were observed in PNEC, CCC & CMC bags at 48 h and the abundance decreased with shift in the species at 72-96 h. The catalase activity in phytoplankton (5.99 nmol/min/ml) and the zooplankton (4.77 nmol/min/ml) showed induction after exposure to PNEC. The present mesocosm study is confirmed that short-term exposure to threshold metal concentration did not affects the phytoplankton community structure in PNEC, but CCC and CMC affects the community structure beyond 24 h. The insights from this study will serve as a baseline information and help develop environmental management tools. We believe that long-term mesocosm experiments would unravel metal detoxification mechanisms at the cellular level and metal transfer rate at higher trophic levels in real-world environment.

Global change alters coastal plankton food webs by promoting the microbial loop: An inverse modelling and network analysis approach on a mesocosm experiment

Marine organisms are currently, and will continue to be, exposed to the simultaneous effects of multiple environmental changes. Plankton organisms form the base of pelagic marine food webs and are particularly sensitive to ecosystem changes. Thus, warming, acidification, and changes in dissolved nutrient concentrations have the potential to alter these assemblages, with consequences for the entire ecosystem. Despite the growing number of studies addressing the potential influence of multiple drivers on plankton, global change may also cause less obvious alterations to the networks of interactions among species. Using inverse analyses applied to data collected during a mesocosm experiment, we aimed to compare the ecological functioning of coastal plankton assemblages and the interactions within their food web under different global change scenarios. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were extended (ERCP) to integrate the future predicted changes in coastal water nutrient concentrations. Overall, we identified that the functioning of the plankton food web was rather similar in the Ambient and ERCP 6.0 scenarios, but substantially altered in the ERCP 8.5 scenario. Using food web modelling and ecological network analysis, we identified that global change strengthens the microbial loop, with a decrease of energy transfer efficiency to higher trophic levels. Microzooplankton responded as well by an increased degree of herbivory in their diet and represented, compared to mesozooplankton, by far the main top-down pressure on primary producers. We also observed that the organisation of the food web and its capacity to recycle carbon was higher under the ERCP 8.5 scenario, but flow diversity and carbon path length were significantly reduced, illustrating an increased food web stability at the expense of diversity. Here, we provide evidence that if global change goes beyond the ERCP 6.0 scenario, coastal ecosystem functioning will be subjected to dramatic changes.

Increased sensitivity of sea urchin larvae to metal toxicity as a consequence of the past two decades of Climate Change and Ocean Acidification in the Mediterranean Sea

The Mediterranean Sea represents a natural laboratory to infer the possible impacts of climate change and ocean acidification. In this article, we report the deteriorating ability of sea urchin larvae (Paracentrotus lividus) to cope with toxicity of a reference contaminant (Cu EC50) over the past 20 years and assessed the influence of 5 environmental factors from satellite measurements. This timeframe was divided in before and after January 2016 (46.57 μg/L vs 28.56 μg/L respectively, p < 0.001). In the second subset of data, correlation of the biological variable with CO2 and pH strengthened compared to the first part (rCO2-EC50: -0.21 vs -0.83 and rpH-EC50: 0.25 vs 0.87 respectively), with a causal link starting from one year and ending 4 months prior to EC50 measurements. Considering the continuous increase in CO2 concentrations recorded recently, this study could reveal a rapid deterioration of the health condition of this population of sea urchins in a coastal ecosystem.

Subtle ecosystem effects of microplastic exposure in marine mesocosms including fish

For two months, communities in 5.8 m³ outdoor marine mesocosms were exposed to 700 μm sphere-shaped polystyrene (PS) beads in dosages between 0.08 and 80 g/m². Barnacle (Semibalanus balanoides) densities were reduced at dosages of 0.8 g/m² onwards without following a standard dose response curve. Lugworms and fish (Solea solea) ingested PS-beads without accumulating them. Lugworms (Arenicola marina) ingested the beads nonselective with the sediment without negative effects. The fish seemed to ingest the plastics only occasionally and at the final sampling day even in the highest dosed mesocosms (>30 beads/cm²) only 20% contained plastic. The condition index of the fish was slightly reduced in mesocosms with dosages of 0.8 g/m² onwards. No difference in condition was found between fish with and without ingested plastic across mesocosms, illustrating the difficulty to relate plastic ingestion with condition from field data. The fish also ingested mollusks with shells exceeding the size of the PS-beads. Bivalves rejected the PS-beads as pseudofeces, without obvious impact on their condition. Mussel's (Mytilus edulis) pseudofeces present an effective matrix to monitor microplastic presence in the water column. Species richness and diversity of the pelagic and benthic community were not affected although, a trend was found that the lower microplastic dosages had a positive effect on the total abundance of benthic invertebrates. In general, the observed effects at even the highest exposure concentrations were that subtle that they will be obscured by natural variation in the field. This underlines the importance of experiments under semi-field conditions for meaningful assessment of the ecological impact of microplastics. This study was performed with the real life, non-toxic, sphere-shaped polystyrene beads as were lost during an actual spill near the Dutch Wadden sea in January 2019. We recommend future mesocosm studies with other types of microplastics, including microfibers, weathered microplastics from sea, and smaller sized particles down to nanoplastics.